Fiber optic networks are being deployed at an ever-increasing rate due, at least in part, to the large bandwidth provided by fiber optic cables. Inherent with any fiber optic network design is the need to connect individual optical fibers to other optical fibers and to equipment. A common technique for connecting optical fibers is by terminating an optical fiber with a ferrule, and bringing the ferrule into a mating relationship with another ferrule terminating a second fiber. The ferrules are precisely aligned by a cylindrical sleeve which receives two ferrules of similar size and coaxially aligns the longitudinal axes of the respective optical cores thereof. The sleeve is typically a component of a coupler or adapter which securely couples together the plug housings of the respective optical fibers. Examples of such connection systems can be found in U.S. Pat. Nos. 4,738,507 and 4,738,508, both issued to Palmquist and assigned to the assignee of the present invention.
Recent advancements have led to the design of smaller optical fiber connection systems which utilize smaller diameter ferrules. The smaller ferrules offer the advantages of being more proportional in size to the fiber optic cable (thereby being more robust due to the reduced mass/weight of the hardware attached at the end of a fiber optic cable), offering improved axial alignment, and being more space efficient. In particular, the relatively smaller connectors can be more densely packed at the face of a piece of network equipment, and therefore, may provide more efficient space management.
The smaller connector design utilizes a ferrule that is 1.25 millimeters (mm) in diameter. An example of a smaller ferrule connector is the LC connector. Prior to the development of the smaller ferrule connectors, most connectors utilized ferrules that were 2.5 mm in diameter, which is twice the diameter of the ferrule of smaller ferrule connectors. An example of the larger ferrule connector is the ST.TM. (a trademark of Lucent Technologies, Inc.) connector.
While there are numerous advantages of using the smaller ferrule connectors, some of which are set forth above, there is a compatibility problem with many of the existing optical networks that utilize the larger ferrule connectors. The larger ferrule connectors have been utilized for several years, and have been expansively deployed in optical networks. It is cost prohibitive in most situations to replace existing optical fiber components with newer components that utilize the smaller ferrule connectors. Therefore, when performing equipment or hardware upgrades or installations to existing optical networks, wherein the new equipment or hardware is designed for use with a smaller ferrule connector and the existing equipment or hardware is designed for use with a larger ferrule connector, the technician is faced with the problem of how to interconnect two optical terminations having ferrules of dissimilar sizes. Thus, a need exists in the industry for an efficient and cost-effective method for interconnecting two different sized ferrules in an optimized mating relationship.
One proposed solution is the use of hybrid optical cables which have a larger ferrule connector on one end to match with the installed network interfaces and a smaller ferrule connector on the opposite end to match with the optical interfaces of the new/upgrade equipment. While such hybrid optical cables provide a relatively straightforward solution, they are relatively cumbersome to use, require two connections in order to interface two optic fibers (i.e., adding an additional connection and the resulting unwanted attenuation and/or other losses), and are relatively expensive.
Yet another proposed solution is a step sleeve adapter which comprises a cylindrical sleeve having coaxially aligned cylindrical bores of different sizes formed into opposite ends of the sleeve and meeting in the middle thereof. Accordingly, at approximately the middle of the step sleeve, the inside diameter of the sleeve, as defined by the bores, changes from a first inside diameter (e.g., corresponding to a smaller ferrule) to a second inside diameter (e.g., corresponding to a larger ferrule). This design requires precise alignment and sizing of the bores, and the precise sizing of the ferrules in order to establish and secure an accurate alignment of the two ferrules received within the sleeve. In practice, such precision is not consistently reproduced economically. For instance, some of the most accurate 1.25 mm ferrules have an outside diameter variance of approximately .+-.0.7 micrometers (.mu.m), and some of the most accurate 2.5 mm ferrules have an outside diameter variance of approximately .+-.0.5 .mu.m. Since the metal or ceramic material comprising the step sleeve is not compliant so as to be able to accommodate variances in either the bore inside diameter or the ferrule outside diameter, the normal and expected variances in the inside bore diameter and the ferrule outside diameter may result in a fit that is not satisfactory, that is, the mechanical stability of the ferrule within the bore may be inadequate for certain applications. For example, a ferrule with an outside diameter less than the outside diameter for which a bore is designed may fit loosely within the bore, allowing transverse loads to misalign the cores of the ferrules within the sleeve. This may result in unwanted and uncontrollable connection loss.
Further, such step sleeves are typically made of a metal which has a high coefficient of thermal expansion thereby making the sleeve particularly sensitive to temperature changes. The step sleeve may be made of a ceramic material in the alternative, which has a more advantageous coefficient of thermal expansion, though a ceramic sleeve would be expensive to manufacture.
Therefore, an unsatisfied need exists in the industry for a low-cost connector for interconnecting optical conductors of dissimilar size with precision alignment of the respective cores thereof.